/**
 * @file
 *
 * Neighbor discovery and stateless address autoconfiguration for IPv6.
 * Aims to be compliant with RFC 4861 (Neighbor discovery) and RFC 4862
 * (Address autoconfiguration).
 */

/*
 * Copyright (c) 2010 Inico Technologies Ltd.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without modification,
 * are permitted provided that the following conditions are met:
 *
 * 1. Redistributions of source code must retain the above copyright notice,
 *    this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright notice,
 *    this list of conditions and the following disclaimer in the documentation
 *    and/or other materials provided with the distribution.
 * 3. The name of the author may not be used to endorse or promote products
 *    derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED
 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT
 * SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT
 * OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
 * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY
 * OF SUCH DAMAGE.
 *
 * This file is part of the lwIP TCP/IP stack.
 *
 * Author: Ivan Delamer <delamer@inicotech.com>
 *
 *
 * Please coordinate changes and requests with Ivan Delamer
 * <delamer@inicotech.com>
 */

#include "lwip/opt.h"

#if LWIP_IPV6  /* don't build if not configured for use in lwipopts.h */

#include "lwip/nd6.h"
#include "lwip/priv/nd6_priv.h"
#include "lwip/prot/nd6.h"
#include "lwip/prot/icmp6.h"
#include "lwip/pbuf.h"
#include "lwip/mem.h"
#include "lwip/memp.h"
#include "lwip/ip6.h"
#include "lwip/ip6_addr.h"
#include "lwip/inet_chksum.h"
#include "lwip/netif.h"
#include "lwip/icmp6.h"
#include "lwip/mld6.h"
#include "lwip/ip.h"
#include "lwip/stats.h"
#include "lwip/dns.h"

#include <string.h>

#ifdef LWIP_HOOK_FILENAME
#include LWIP_HOOK_FILENAME
#endif

#if LWIP_IPV6_DUP_DETECT_ATTEMPTS > IP6_ADDR_TENTATIVE_COUNT_MASK
#error LWIP_IPV6_DUP_DETECT_ATTEMPTS > IP6_ADDR_TENTATIVE_COUNT_MASK
#endif

/* Router tables. */
struct nd6_neighbor_cache_entry neighbor_cache[LWIP_ND6_NUM_NEIGHBORS];
struct nd6_destination_cache_entry destination_cache[LWIP_ND6_NUM_DESTINATIONS];
struct nd6_prefix_list_entry prefix_list[LWIP_ND6_NUM_PREFIXES];
struct nd6_router_list_entry default_router_list[LWIP_ND6_NUM_ROUTERS];

/* Default values, can be updated by a RA message. */
u32_t reachable_time = LWIP_ND6_REACHABLE_TIME;
u32_t retrans_timer = LWIP_ND6_RETRANS_TIMER; /* @todo implement this value in timer */

/* Index for cache entries. */
static u8_t nd6_cached_neighbor_index;
static u8_t nd6_cached_destination_index;

/* Multicast address holder. */
static ip6_addr_t multicast_address;

/* Static buffer to parse RA packet options (size of a prefix option, biggest option) */
static u8_t nd6_ra_buffer[sizeof(struct prefix_option)];

/* Forward declarations. */
static s8_t nd6_find_neighbor_cache_entry(const ip6_addr_t *ip6addr);
static s8_t nd6_new_neighbor_cache_entry(void);
static void nd6_free_neighbor_cache_entry(s8_t i);
static s8_t nd6_find_destination_cache_entry(const ip6_addr_t *ip6addr);
static s8_t nd6_new_destination_cache_entry(void);
static s8_t nd6_is_prefix_in_netif(const ip6_addr_t *ip6addr, struct netif *netif);
static s8_t nd6_select_router(const ip6_addr_t *ip6addr, struct netif *netif);
static s8_t nd6_get_router(const ip6_addr_t *router_addr, struct netif *netif);
static s8_t nd6_new_router(const ip6_addr_t *router_addr, struct netif *netif);
static s8_t nd6_get_onlink_prefix(ip6_addr_t *prefix, struct netif *netif);
static s8_t nd6_new_onlink_prefix(ip6_addr_t *prefix, struct netif *netif);
static s8_t nd6_get_next_hop_entry(const ip6_addr_t *ip6addr, struct netif *netif);
static err_t nd6_queue_packet(s8_t neighbor_index, struct pbuf *q);

#define ND6_SEND_FLAG_MULTICAST_DEST 0x01
#define ND6_SEND_FLAG_ALLNODES_DEST 0x02
static void nd6_send_ns(struct netif *netif, const ip6_addr_t *target_addr, u8_t flags);
static void nd6_send_na(struct netif *netif, const ip6_addr_t *target_addr, u8_t flags);
static void nd6_send_neighbor_cache_probe(struct nd6_neighbor_cache_entry *entry, u8_t flags);
#if LWIP_IPV6_SEND_ROUTER_SOLICIT
static err_t nd6_send_rs(struct netif *netif);
#endif /* LWIP_IPV6_SEND_ROUTER_SOLICIT */

#if LWIP_ND6_QUEUEING
static void nd6_free_q(struct nd6_q_entry *q);
#else /* LWIP_ND6_QUEUEING */
#define nd6_free_q(q) pbuf_free(q)
#endif /* LWIP_ND6_QUEUEING */
static void nd6_send_q(s8_t i);


/**
 * Process an incoming neighbor discovery message
 *
 * @param p the nd packet, p->payload pointing to the icmpv6 header
 * @param inp the netif on which this packet was received
 */
void
nd6_input(struct pbuf *p, struct netif *inp)
{
    u8_t msg_type;
    s8_t i;

    ND6_STATS_INC(nd6.recv);

    msg_type = *((u8_t *)p->payload);
    switch (msg_type)
    {
    case ICMP6_TYPE_NA: /* Neighbor Advertisement. */
    {
        struct na_header *na_hdr;
        struct lladdr_option *lladdr_opt;

        /* Check that na header fits in packet. */
        if (p->len < (sizeof(struct na_header)))
        {
            /* @todo debug message */
            pbuf_free(p);
            ND6_STATS_INC(nd6.lenerr);
            ND6_STATS_INC(nd6.drop);
            return;
        }

        na_hdr = (struct na_header *)p->payload;

        /* Unsolicited NA?*/
        if (ip6_addr_ismulticast(ip6_current_dest_addr()))
        {
            ip6_addr_t target_address;

            /* This is an unsolicited NA.
             * link-layer changed?
             * part of DAD mechanism? */

            /* Create an aligned copy. */
            ip6_addr_set(&target_address, &(na_hdr->target_address));

#if LWIP_IPV6_DUP_DETECT_ATTEMPTS
            /* If the target address matches this netif, it is a DAD response. */
            for (i = 0; i < LWIP_IPV6_NUM_ADDRESSES; i++)
            {
                if (!ip6_addr_isinvalid(netif_ip6_addr_state(inp, i)) &&
                        ip6_addr_cmp(&target_address, netif_ip6_addr(inp, i)))
                {
                    /* We are using a duplicate address. */
                    netif_ip6_addr_set_state(inp, i, IP6_ADDR_INVALID);

#if LWIP_IPV6_AUTOCONFIG
                    /* Check to see if this address was autoconfigured. */
                    if (!ip6_addr_islinklocal(&target_address))
                    {
                        i = nd6_get_onlink_prefix(&target_address, inp);
                        if (i >= 0)
                        {
                            /* Mark this prefix as duplicate, so that we don't use it
                             * to generate this address again. */
                            prefix_list[i].flags |= ND6_PREFIX_AUTOCONFIG_ADDRESS_DUPLICATE;
                        }
                    }
#endif /* LWIP_IPV6_AUTOCONFIG */

                    pbuf_free(p);
                    return;
                }
            }
#endif /* LWIP_IPV6_DUP_DETECT_ATTEMPTS */

            /* Check that link-layer address option also fits in packet. */
            if (p->len < (sizeof(struct na_header) + 2))
            {
                /* @todo debug message */
                pbuf_free(p);
                ND6_STATS_INC(nd6.lenerr);
                ND6_STATS_INC(nd6.drop);
                return;
            }

            lladdr_opt = (struct lladdr_option *)((u8_t *)p->payload + sizeof(struct na_header));

            if (p->len < (sizeof(struct na_header) + (lladdr_opt->length << 3)))
            {
                /* @todo debug message */
                pbuf_free(p);
                ND6_STATS_INC(nd6.lenerr);
                ND6_STATS_INC(nd6.drop);
                return;
            }

            /* This is an unsolicited NA, most likely there was a LLADDR change. */
            i = nd6_find_neighbor_cache_entry(&target_address);
            if (i >= 0)
            {
                if (na_hdr->flags & ND6_FLAG_OVERRIDE)
                {
                    MEMCPY(neighbor_cache[i].lladdr, lladdr_opt->addr, inp->hwaddr_len);
                }
            }
        }
        else
        {
            ip6_addr_t target_address;

            /* This is a solicited NA.
             * neighbor address resolution response?
             * neighbor unreachability detection response? */

            /* Create an aligned copy. */
            ip6_addr_set(&target_address, &(na_hdr->target_address));

            /* Find the cache entry corresponding to this na. */
            i = nd6_find_neighbor_cache_entry(&target_address);
            if (i < 0)
            {
                /* We no longer care about this target address. drop it. */
                pbuf_free(p);
                return;
            }

            /* Update cache entry. */
            if ((na_hdr->flags & ND6_FLAG_OVERRIDE) ||
                    (neighbor_cache[i].state == ND6_INCOMPLETE))
            {
                /* Check that link-layer address option also fits in packet. */
                if (p->len < (sizeof(struct na_header) + 2))
                {
                    /* @todo debug message */
                    pbuf_free(p);
                    ND6_STATS_INC(nd6.lenerr);
                    ND6_STATS_INC(nd6.drop);
                    return;
                }

                lladdr_opt = (struct lladdr_option *)((u8_t *)p->payload + sizeof(struct na_header));

                if (p->len < (sizeof(struct na_header) + (lladdr_opt->length << 3)))
                {
                    /* @todo debug message */
                    pbuf_free(p);
                    ND6_STATS_INC(nd6.lenerr);
                    ND6_STATS_INC(nd6.drop);
                    return;
                }

                MEMCPY(neighbor_cache[i].lladdr, lladdr_opt->addr, inp->hwaddr_len);
            }

            neighbor_cache[i].netif = inp;
            neighbor_cache[i].state = ND6_REACHABLE;
            neighbor_cache[i].counter.reachable_time = reachable_time;

            /* Send queued packets, if any. */
            if (neighbor_cache[i].q != NULL)
            {
                nd6_send_q(i);
            }
        }

        break; /* ICMP6_TYPE_NA */
    }
    case ICMP6_TYPE_NS: /* Neighbor solicitation. */
    {
        struct ns_header *ns_hdr;
        struct lladdr_option *lladdr_opt;
        u8_t accepted;

        /* Check that ns header fits in packet. */
        if (p->len < sizeof(struct ns_header))
        {
            /* @todo debug message */
            pbuf_free(p);
            ND6_STATS_INC(nd6.lenerr);
            ND6_STATS_INC(nd6.drop);
            return;
        }

        ns_hdr = (struct ns_header *)p->payload;

        /* Check if there is a link-layer address provided. Only point to it if in this buffer. */
        if (p->len >= (sizeof(struct ns_header) + 2))
        {
            lladdr_opt = (struct lladdr_option *)((u8_t *)p->payload + sizeof(struct ns_header));
            if (p->len < (sizeof(struct ns_header) + (lladdr_opt->length << 3)))
            {
                lladdr_opt = NULL;
            }
        }
        else
        {
            lladdr_opt = NULL;
        }

        /* Check if the target address is configured on the receiving netif. */
        accepted = 0;
        for (i = 0; i < LWIP_IPV6_NUM_ADDRESSES; ++i)
        {
            if ((ip6_addr_isvalid(netif_ip6_addr_state(inp, i)) ||
                    (ip6_addr_istentative(netif_ip6_addr_state(inp, i)) &&
                     ip6_addr_isany(ip6_current_src_addr()))) &&
                    ip6_addr_cmp(&(ns_hdr->target_address), netif_ip6_addr(inp, i)))
            {
                accepted = 1;
                break;
            }
        }

        /* NS not for us? */
        if (!accepted)
        {
            pbuf_free(p);
            return;
        }

        /* Check for ANY address in src (DAD algorithm). */
        if (ip6_addr_isany(ip6_current_src_addr()))
        {
            /* Sender is validating this address. */
            for (i = 0; i < LWIP_IPV6_NUM_ADDRESSES; ++i)
            {
                if (!ip6_addr_isinvalid(netif_ip6_addr_state(inp, i)) &&
                        ip6_addr_cmp(&(ns_hdr->target_address), netif_ip6_addr(inp, i)))
                {
                    /* Send a NA back so that the sender does not use this address. */
                    nd6_send_na(inp, netif_ip6_addr(inp, i), ND6_FLAG_OVERRIDE | ND6_SEND_FLAG_ALLNODES_DEST);
                    if (ip6_addr_istentative(netif_ip6_addr_state(inp, i)))
                    {
                        /* We shouldn't use this address either. */
                        netif_ip6_addr_set_state(inp, i, IP6_ADDR_INVALID);
                    }
                }
            }
        }
        else
        {
            ip6_addr_t target_address;

            /* Sender is trying to resolve our address. */
            /* Verify that they included their own link-layer address. */
            if (lladdr_opt == NULL)
            {
                /* Not a valid message. */
                pbuf_free(p);
                ND6_STATS_INC(nd6.proterr);
                ND6_STATS_INC(nd6.drop);
                return;
            }

            i = nd6_find_neighbor_cache_entry(ip6_current_src_addr());
            if (i >= 0)
            {
                /* We already have a record for the solicitor. */
                if (neighbor_cache[i].state == ND6_INCOMPLETE)
                {
                    neighbor_cache[i].netif = inp;
                    MEMCPY(neighbor_cache[i].lladdr, lladdr_opt->addr, inp->hwaddr_len);

                    /* Delay probe in case we get confirmation of reachability from upper layer (TCP). */
                    neighbor_cache[i].state = ND6_DELAY;
                    neighbor_cache[i].counter.delay_time = LWIP_ND6_DELAY_FIRST_PROBE_TIME / ND6_TMR_INTERVAL;
                }
            }
            else
            {
                /* Add their IPv6 address and link-layer address to neighbor cache.
                 * We will need it at least to send a unicast NA message, but most
                 * likely we will also be communicating with this node soon. */
                i = nd6_new_neighbor_cache_entry();
                if (i < 0)
                {
                    /* We couldn't assign a cache entry for this neighbor.
                     * we won't be able to reply. drop it. */
                    pbuf_free(p);
                    ND6_STATS_INC(nd6.memerr);
                    return;
                }
                neighbor_cache[i].netif = inp;
                MEMCPY(neighbor_cache[i].lladdr, lladdr_opt->addr, inp->hwaddr_len);
                ip6_addr_set(&(neighbor_cache[i].next_hop_address), ip6_current_src_addr());

                /* Receiving a message does not prove reachability: only in one direction.
                 * Delay probe in case we get confirmation of reachability from upper layer (TCP). */
                neighbor_cache[i].state = ND6_DELAY;
                neighbor_cache[i].counter.delay_time = LWIP_ND6_DELAY_FIRST_PROBE_TIME / ND6_TMR_INTERVAL;
            }

            /* Create an aligned copy. */
            ip6_addr_set(&target_address, &(ns_hdr->target_address));

            /* Send back a NA for us. Allocate the reply pbuf. */
            nd6_send_na(inp, &target_address, ND6_FLAG_SOLICITED | ND6_FLAG_OVERRIDE);
        }

        break; /* ICMP6_TYPE_NS */
    }
    case ICMP6_TYPE_RA: /* Router Advertisement. */
    {
        struct ra_header *ra_hdr;
        u8_t *buffer; /* Used to copy options. */
        u16_t offset;
#if LWIP_ND6_RDNSS_MAX_DNS_SERVERS
        /* There can by multiple RDNSS options per RA */
        u8_t rdnss_server_idx = 0;
#endif /* LWIP_ND6_RDNSS_MAX_DNS_SERVERS */

        /* Check that RA header fits in packet. */
        if (p->len < sizeof(struct ra_header))
        {
            /* @todo debug message */
            pbuf_free(p);
            ND6_STATS_INC(nd6.lenerr);
            ND6_STATS_INC(nd6.drop);
            return;
        }

        ra_hdr = (struct ra_header *)p->payload;

        /* If we are sending RS messages, stop. */
#if LWIP_IPV6_SEND_ROUTER_SOLICIT
        /* ensure at least one solicitation is sent */
        if ((inp->rs_count < LWIP_ND6_MAX_MULTICAST_SOLICIT) ||
                (nd6_send_rs(inp) == ERR_OK))
        {
            inp->rs_count = 0;
        }
#endif /* LWIP_IPV6_SEND_ROUTER_SOLICIT */

        /* Get the matching default router entry. */
        i = nd6_get_router(ip6_current_src_addr(), inp);
        if (i < 0)
        {
            /* Create a new router entry. */
            i = nd6_new_router(ip6_current_src_addr(), inp);
        }

        if (i < 0)
        {
            /* Could not create a new router entry. */
            pbuf_free(p);
            ND6_STATS_INC(nd6.memerr);
            return;
        }

        /* Re-set invalidation timer. */
        default_router_list[i].invalidation_timer = lwip_htons(ra_hdr->router_lifetime);

        /* Re-set default timer values. */
#if LWIP_ND6_ALLOW_RA_UPDATES
        if (ra_hdr->retrans_timer > 0)
        {
            retrans_timer = lwip_htonl(ra_hdr->retrans_timer);
        }
        if (ra_hdr->reachable_time > 0)
        {
            reachable_time = lwip_htonl(ra_hdr->reachable_time);
        }
#endif /* LWIP_ND6_ALLOW_RA_UPDATES */

        /* @todo set default hop limit... */
        /* ra_hdr->current_hop_limit;*/

        /* Update flags in local entry (incl. preference). */
        default_router_list[i].flags = ra_hdr->flags;

        /* Offset to options. */
        offset = sizeof(struct ra_header);

        /* Process each option. */
        while ((p->tot_len - offset) > 0)
        {
            if (p->len == p->tot_len)
            {
                /* no need to copy from contiguous pbuf */
                buffer = &((u8_t *)p->payload)[offset];
            }
            else
            {
                buffer = nd6_ra_buffer;
                if (pbuf_copy_partial(p, buffer, sizeof(struct prefix_option), offset) != sizeof(struct prefix_option))
                {
                    pbuf_free(p);
                    ND6_STATS_INC(nd6.lenerr);
                    ND6_STATS_INC(nd6.drop);
                    return;
                }
            }
            if (buffer[1] == 0)
            {
                /* zero-length extension. drop packet */
                pbuf_free(p);
                ND6_STATS_INC(nd6.lenerr);
                ND6_STATS_INC(nd6.drop);
                return;
            }
            switch (buffer[0])
            {
            case ND6_OPTION_TYPE_SOURCE_LLADDR:
            {
                struct lladdr_option *lladdr_opt;
                lladdr_opt = (struct lladdr_option *)buffer;
                if ((default_router_list[i].neighbor_entry != NULL) &&
                        (default_router_list[i].neighbor_entry->state == ND6_INCOMPLETE))
                {
                    SMEMCPY(default_router_list[i].neighbor_entry->lladdr, lladdr_opt->addr, inp->hwaddr_len);
                    default_router_list[i].neighbor_entry->state = ND6_REACHABLE;
                    default_router_list[i].neighbor_entry->counter.reachable_time = reachable_time;
                }
                break;
            }
            case ND6_OPTION_TYPE_MTU:
            {
                struct mtu_option *mtu_opt;
                mtu_opt = (struct mtu_option *)buffer;
                if (lwip_htonl(mtu_opt->mtu) >= 1280)
                {
#if LWIP_ND6_ALLOW_RA_UPDATES
                    inp->mtu = (u16_t)lwip_htonl(mtu_opt->mtu);
#endif /* LWIP_ND6_ALLOW_RA_UPDATES */
                }
                break;
            }
            case ND6_OPTION_TYPE_PREFIX_INFO:
            {
                struct prefix_option *prefix_opt;
                prefix_opt = (struct prefix_option *)buffer;

                if ((prefix_opt->flags & ND6_PREFIX_FLAG_ON_LINK) &&
                        (prefix_opt->prefix_length == 64)  &&
                        !ip6_addr_islinklocal(&(prefix_opt->prefix)))
                {
                    /* Add to on-link prefix list. */
                    s8_t prefix;
                    ip6_addr_t prefix_addr;

                    /* Get a memory-aligned copy of the prefix. */
                    ip6_addr_set(&prefix_addr, &(prefix_opt->prefix));

                    /* find cache entry for this prefix. */
                    prefix = nd6_get_onlink_prefix(&prefix_addr, inp);
                    if (prefix < 0)
                    {
                        /* Create a new cache entry. */
                        prefix = nd6_new_onlink_prefix(&prefix_addr, inp);
                    }
                    if (prefix >= 0)
                    {
                        prefix_list[prefix].invalidation_timer = lwip_htonl(prefix_opt->valid_lifetime);

#if LWIP_IPV6_AUTOCONFIG
                        if (prefix_opt->flags & ND6_PREFIX_FLAG_AUTONOMOUS)
                        {
                            /* Mark prefix as autonomous, so that address autoconfiguration can take place.
                             * Only OR flag, so that we don't over-write other flags (such as ADDRESS_DUPLICATE)*/
                            prefix_list[prefix].flags |= ND6_PREFIX_AUTOCONFIG_AUTONOMOUS;
                        }
#endif /* LWIP_IPV6_AUTOCONFIG */
                    }
                }

                break;
            }
            case ND6_OPTION_TYPE_ROUTE_INFO:
                /* @todo implement preferred routes.
                struct route_option * route_opt;
                route_opt = (struct route_option *)buffer;*/

                break;
#if LWIP_ND6_RDNSS_MAX_DNS_SERVERS
            case ND6_OPTION_TYPE_RDNSS:
            {
                u8_t num, n;
                struct rdnss_option *rdnss_opt;

                rdnss_opt = (struct rdnss_option *)buffer;
                num = (rdnss_opt->length - 1) / 2;
                for (n = 0; (rdnss_server_idx < DNS_MAX_SERVERS) && (n < num); n++)
                {
                    ip_addr_t rdnss_address;

                    /* Get a memory-aligned copy of the prefix. */
                    ip_addr_copy_from_ip6(rdnss_address, rdnss_opt->rdnss_address[n]);

                    if (htonl(rdnss_opt->lifetime) > 0)
                    {
                        /* TODO implement Lifetime > 0 */
                        dns_setserver(rdnss_server_idx++, &rdnss_address);
                    }
                    else
                    {
                        /* TODO implement DNS removal in dns.c */
                        u8_t s;
                        for (s = 0; s < DNS_MAX_SERVERS; s++)
                        {
                            const ip_addr_t *addr = dns_getserver(s);
                            if(ip_addr_cmp(addr, &rdnss_address))
                            {
                                dns_setserver(s, NULL);
                            }
                        }
                    }
                }
                break;
            }
#endif /* LWIP_ND6_RDNSS_MAX_DNS_SERVERS */
            default:
                /* Unrecognized option, abort. */
                ND6_STATS_INC(nd6.proterr);
                break;
            }
            /* option length is checked earlier to be non-zero to make sure loop ends */
            offset += 8 * ((u16_t)buffer[1]);
        }

        break; /* ICMP6_TYPE_RA */
    }
    case ICMP6_TYPE_RD: /* Redirect */
    {
        struct redirect_header *redir_hdr;
        struct lladdr_option *lladdr_opt;
        ip6_addr_t tmp;

        /* Check that Redir header fits in packet. */
        if (p->len < sizeof(struct redirect_header))
        {
            /* @todo debug message */
            pbuf_free(p);
            ND6_STATS_INC(nd6.lenerr);
            ND6_STATS_INC(nd6.drop);
            return;
        }

        redir_hdr = (struct redirect_header *)p->payload;

        if (p->len >= (sizeof(struct redirect_header) + 2))
        {
            lladdr_opt = (struct lladdr_option *)((u8_t *)p->payload + sizeof(struct redirect_header));
            if (p->len < (sizeof(struct redirect_header) + (lladdr_opt->length << 3)))
            {
                lladdr_opt = NULL;
            }
        }
        else
        {
            lladdr_opt = NULL;
        }

        /* Copy original destination address to current source address, to have an aligned copy. */
        ip6_addr_set(&tmp, &(redir_hdr->destination_address));

        /* Find dest address in cache */
        i = nd6_find_destination_cache_entry(&tmp);
        if (i < 0)
        {
            /* Destination not in cache, drop packet. */
            pbuf_free(p);
            return;
        }

        /* Set the new target address. */
        ip6_addr_set(&(destination_cache[i].next_hop_addr), &(redir_hdr->target_address));

        /* If Link-layer address of other router is given, try to add to neighbor cache. */
        if (lladdr_opt != NULL)
        {
            if (lladdr_opt->type == ND6_OPTION_TYPE_TARGET_LLADDR)
            {
                /* Copy target address to current source address, to have an aligned copy. */
                ip6_addr_set(&tmp, &(redir_hdr->target_address));

                i = nd6_find_neighbor_cache_entry(&tmp);
                if (i < 0)
                {
                    i = nd6_new_neighbor_cache_entry();
                    if (i >= 0)
                    {
                        neighbor_cache[i].netif = inp;
                        MEMCPY(neighbor_cache[i].lladdr, lladdr_opt->addr, inp->hwaddr_len);
                        ip6_addr_set(&(neighbor_cache[i].next_hop_address), &tmp);

                        /* Receiving a message does not prove reachability: only in one direction.
                         * Delay probe in case we get confirmation of reachability from upper layer (TCP). */
                        neighbor_cache[i].state = ND6_DELAY;
                        neighbor_cache[i].counter.delay_time = LWIP_ND6_DELAY_FIRST_PROBE_TIME / ND6_TMR_INTERVAL;
                    }
                }
                if (i >= 0)
                {
                    if (neighbor_cache[i].state == ND6_INCOMPLETE)
                    {
                        MEMCPY(neighbor_cache[i].lladdr, lladdr_opt->addr, inp->hwaddr_len);
                        /* Receiving a message does not prove reachability: only in one direction.
                         * Delay probe in case we get confirmation of reachability from upper layer (TCP). */
                        neighbor_cache[i].state = ND6_DELAY;
                        neighbor_cache[i].counter.delay_time = LWIP_ND6_DELAY_FIRST_PROBE_TIME / ND6_TMR_INTERVAL;
                    }
                }
            }
        }
        break; /* ICMP6_TYPE_RD */
    }
    case ICMP6_TYPE_PTB: /* Packet too big */
    {
        struct icmp6_hdr *icmp6hdr; /* Packet too big message */
        struct ip6_hdr *ip6hdr; /* IPv6 header of the packet which caused the error */
        u32_t pmtu;
        ip6_addr_t tmp;

        /* Check that ICMPv6 header + IPv6 header fit in payload */
        if (p->len < (sizeof(struct icmp6_hdr) + IP6_HLEN))
        {
            /* drop short packets */
            pbuf_free(p);
            ND6_STATS_INC(nd6.lenerr);
            ND6_STATS_INC(nd6.drop);
            return;
        }

        icmp6hdr = (struct icmp6_hdr *)p->payload;
        ip6hdr = (struct ip6_hdr *)((u8_t *)p->payload + sizeof(struct icmp6_hdr));

        /* Copy original destination address to current source address, to have an aligned copy. */
        ip6_addr_set(&tmp, &(ip6hdr->dest));

        /* Look for entry in destination cache. */
        i = nd6_find_destination_cache_entry(&tmp);
        if (i < 0)
        {
            /* Destination not in cache, drop packet. */
            pbuf_free(p);
            return;
        }

        /* Change the Path MTU. */
        pmtu = lwip_htonl(icmp6hdr->data);
        destination_cache[i].pmtu = (u16_t)LWIP_MIN(pmtu, 0xFFFF);

        break; /* ICMP6_TYPE_PTB */
    }

    default:
        ND6_STATS_INC(nd6.proterr);
        ND6_STATS_INC(nd6.drop);
        break; /* default */
    }

    pbuf_free(p);
}


/**
 * Periodic timer for Neighbor discovery functions:
 *
 * - Update neighbor reachability states
 * - Update destination cache entries age
 * - Update invalidation timers of default routers and on-link prefixes
 * - Perform duplicate address detection (DAD) for our addresses
 * - Send router solicitations
 */
void
nd6_tmr(void)
{
    s8_t i;
    struct netif *netif;

    /* Process neighbor entries. */
    for (i = 0; i < LWIP_ND6_NUM_NEIGHBORS; i++)
    {
        switch (neighbor_cache[i].state)
        {
        case ND6_INCOMPLETE:
            if ((neighbor_cache[i].counter.probes_sent >= LWIP_ND6_MAX_MULTICAST_SOLICIT) &&
                    (!neighbor_cache[i].isrouter))
            {
                /* Retries exceeded. */
                nd6_free_neighbor_cache_entry(i);
            }
            else
            {
                /* Send a NS for this entry. */
                neighbor_cache[i].counter.probes_sent++;
                nd6_send_neighbor_cache_probe(&neighbor_cache[i], ND6_SEND_FLAG_MULTICAST_DEST);
            }
            break;
        case ND6_REACHABLE:
            /* Send queued packets, if any are left. Should have been sent already. */
            if (neighbor_cache[i].q != NULL)
            {
                nd6_send_q(i);
            }
            if (neighbor_cache[i].counter.reachable_time <= ND6_TMR_INTERVAL)
            {
                /* Change to stale state. */
                neighbor_cache[i].state = ND6_STALE;
                neighbor_cache[i].counter.stale_time = 0;
            }
            else
            {
                neighbor_cache[i].counter.reachable_time -= ND6_TMR_INTERVAL;
            }
            break;
        case ND6_STALE:
            neighbor_cache[i].counter.stale_time++;
            break;
        case ND6_DELAY:
            if (neighbor_cache[i].counter.delay_time <= 1)
            {
                /* Change to PROBE state. */
                neighbor_cache[i].state = ND6_PROBE;
                neighbor_cache[i].counter.probes_sent = 0;
            }
            else
            {
                neighbor_cache[i].counter.delay_time--;
            }
            break;
        case ND6_PROBE:
            if ((neighbor_cache[i].counter.probes_sent >= LWIP_ND6_MAX_MULTICAST_SOLICIT) &&
                    (!neighbor_cache[i].isrouter))
            {
                /* Retries exceeded. */
                nd6_free_neighbor_cache_entry(i);
            }
            else
            {
                /* Send a NS for this entry. */
                neighbor_cache[i].counter.probes_sent++;
                nd6_send_neighbor_cache_probe(&neighbor_cache[i], 0);
            }
            break;
        case ND6_NO_ENTRY:
        default:
            /* Do nothing. */
            break;
        }
    }

    /* Process destination entries. */
    for (i = 0; i < LWIP_ND6_NUM_DESTINATIONS; i++)
    {
        destination_cache[i].age++;
    }

    /* Process router entries. */
    for (i = 0; i < LWIP_ND6_NUM_ROUTERS; i++)
    {
        if (default_router_list[i].neighbor_entry != NULL)
        {
            /* Active entry. */
            if (default_router_list[i].invalidation_timer > 0)
            {
                default_router_list[i].invalidation_timer -= ND6_TMR_INTERVAL / 1000;
            }
            if (default_router_list[i].invalidation_timer < ND6_TMR_INTERVAL / 1000)
            {
                /* Less than 1 second remaining. Clear this entry. */
                default_router_list[i].neighbor_entry->isrouter = 0;
                default_router_list[i].neighbor_entry = NULL;
                default_router_list[i].invalidation_timer = 0;
                default_router_list[i].flags = 0;
            }
        }
    }

    /* Process prefix entries. */
    for (i = 0; i < LWIP_ND6_NUM_PREFIXES; i++)
    {
        if (prefix_list[i].netif != NULL)
        {
            if (prefix_list[i].invalidation_timer < ND6_TMR_INTERVAL / 1000)
            {
                /* Entry timed out, remove it */
                prefix_list[i].invalidation_timer = 0;

#if LWIP_IPV6_AUTOCONFIG
                /* If any addresses were configured with this prefix, remove them */
                if (prefix_list[i].flags & ND6_PREFIX_AUTOCONFIG_ADDRESS_GENERATED)
                {
                    s8_t j;

                    for (j = 1; j < LWIP_IPV6_NUM_ADDRESSES; j++)
                    {
                        if ((netif_ip6_addr_state(prefix_list[i].netif, j) != IP6_ADDR_INVALID) &&
                                ip6_addr_netcmp(&prefix_list[i].prefix, netif_ip6_addr(prefix_list[i].netif, j)))
                        {
                            netif_ip6_addr_set_state(prefix_list[i].netif, j, IP6_ADDR_INVALID);
                            prefix_list[i].flags = 0;

                            /* Exit loop. */
                            break;
                        }
                    }
                }
#endif /* LWIP_IPV6_AUTOCONFIG */

                prefix_list[i].netif = NULL;
                prefix_list[i].flags = 0;
            }
            else
            {
                prefix_list[i].invalidation_timer -= ND6_TMR_INTERVAL / 1000;

#if LWIP_IPV6_AUTOCONFIG
                /* Initiate address autoconfiguration for this prefix, if conditions are met. */
                if (prefix_list[i].netif->ip6_autoconfig_enabled &&
                        (prefix_list[i].flags & ND6_PREFIX_AUTOCONFIG_AUTONOMOUS) &&
                        !(prefix_list[i].flags & ND6_PREFIX_AUTOCONFIG_ADDRESS_GENERATED))
                {
                    s8_t j;
                    /* Try to get an address on this netif that is invalid.
                     * Skip 0 index (link-local address) */
                    for (j = 1; j < LWIP_IPV6_NUM_ADDRESSES; j++)
                    {
                        if (netif_ip6_addr_state(prefix_list[i].netif, j) == IP6_ADDR_INVALID)
                        {
                            /* Generate an address using this prefix and interface ID from link-local address. */
                            netif_ip6_addr_set_parts(prefix_list[i].netif, j,
                                                     prefix_list[i].prefix.addr[0], prefix_list[i].prefix.addr[1],
                                                     netif_ip6_addr(prefix_list[i].netif, 0)->addr[2], netif_ip6_addr(prefix_list[i].netif, 0)->addr[3]);

                            /* Mark it as tentative (DAD will be performed if configured). */
                            netif_ip6_addr_set_state(prefix_list[i].netif, j, IP6_ADDR_TENTATIVE);

                            /* Mark this prefix with ADDRESS_GENERATED, so that we don't try again. */
                            prefix_list[i].flags |= ND6_PREFIX_AUTOCONFIG_ADDRESS_GENERATED;

                            /* Exit loop. */
                            break;
                        }
                    }
                }
#endif /* LWIP_IPV6_AUTOCONFIG */
            }
        }
    }


    /* Process our own addresses, if DAD configured. */
    for (netif = netif_list; netif != NULL; netif = netif->next)
    {
        for (i = 0; i < LWIP_IPV6_NUM_ADDRESSES; ++i)
        {
            u8_t addr_state = netif_ip6_addr_state(netif, i);
            if (ip6_addr_istentative(addr_state))
            {
                if ((addr_state & IP6_ADDR_TENTATIVE_COUNT_MASK) >= LWIP_IPV6_DUP_DETECT_ATTEMPTS)
                {
                    /* No NA received in response. Mark address as valid. */
                    netif_ip6_addr_set_state(netif, i, IP6_ADDR_PREFERRED);
                    /* @todo implement preferred and valid lifetimes. */
                }
                else if (netif->flags & NETIF_FLAG_UP)
                {
                    /* Send a NS for this address. */
                    nd6_send_ns(netif, netif_ip6_addr(netif, i), ND6_SEND_FLAG_MULTICAST_DEST);
                    /* tentative: set next state by increasing by one */
                    netif_ip6_addr_set_state(netif, i, addr_state + 1);
                    /* @todo send max 1 NS per tmr call? enable return*/
                    /*return;*/
                }
            }
        }
    }

#if LWIP_IPV6_SEND_ROUTER_SOLICIT
    /* Send router solicitation messages, if necessary. */
    for (netif = netif_list; netif != NULL; netif = netif->next)
    {
        if ((netif->rs_count > 0) && (netif->flags & NETIF_FLAG_UP) &&
                (!ip6_addr_isinvalid(netif_ip6_addr_state(netif, 0))))
        {
            if (nd6_send_rs(netif) == ERR_OK)
            {
                netif->rs_count--;
            }
        }
    }
#endif /* LWIP_IPV6_SEND_ROUTER_SOLICIT */

}

/** Send a neighbor solicitation message for a specific neighbor cache entry
 *
 * @param entry the neightbor cache entry for wich to send the message
 * @param flags one of ND6_SEND_FLAG_*
 */
static void
nd6_send_neighbor_cache_probe(struct nd6_neighbor_cache_entry *entry, u8_t flags)
{
    nd6_send_ns(entry->netif, &entry->next_hop_address, flags);
}

/**
 * Send a neighbor solicitation message
 *
 * @param netif the netif on which to send the message
 * @param target_addr the IPv6 target address for the ND message
 * @param flags one of ND6_SEND_FLAG_*
 */
static void
nd6_send_ns(struct netif *netif, const ip6_addr_t *target_addr, u8_t flags)
{
    struct ns_header *ns_hdr;
    struct pbuf *p;
    const ip6_addr_t *src_addr;
    u16_t lladdr_opt_len;

    if (ip6_addr_isvalid(netif_ip6_addr_state(netif, 0)))
    {
        /* Use link-local address as source address. */
        src_addr = netif_ip6_addr(netif, 0);
        /* calculate option length (in 8-byte-blocks) */
        lladdr_opt_len = ((netif->hwaddr_len + 2) + 7) >> 3;
    }
    else
    {
        src_addr = IP6_ADDR_ANY6;
        /* Option "MUST NOT be included when the source IP address is the unspecified address." */
        lladdr_opt_len = 0;
    }

    /* Allocate a packet. */
    p = pbuf_alloc(PBUF_IP, sizeof(struct ns_header) + (lladdr_opt_len << 3), PBUF_RAM);
    if (p == NULL)
    {
        ND6_STATS_INC(nd6.memerr);
        return;
    }

    /* Set fields. */
    ns_hdr = (struct ns_header *)p->payload;

    ns_hdr->type = ICMP6_TYPE_NS;
    ns_hdr->code = 0;
    ns_hdr->chksum = 0;
    ns_hdr->reserved = 0;
    ip6_addr_set(&(ns_hdr->target_address), target_addr);

    if (lladdr_opt_len != 0)
    {
        struct lladdr_option *lladdr_opt = (struct lladdr_option *)((u8_t *)p->payload + sizeof(struct ns_header));
        lladdr_opt->type = ND6_OPTION_TYPE_SOURCE_LLADDR;
        lladdr_opt->length = (u8_t)lladdr_opt_len;
        SMEMCPY(lladdr_opt->addr, netif->hwaddr, netif->hwaddr_len);
    }

    /* Generate the solicited node address for the target address. */
    if (flags & ND6_SEND_FLAG_MULTICAST_DEST)
    {
        ip6_addr_set_solicitednode(&multicast_address, target_addr->addr[3]);
        target_addr = &multicast_address;
    }

#if CHECKSUM_GEN_ICMP6
    IF__NETIF_CHECKSUM_ENABLED(netif, NETIF_CHECKSUM_GEN_ICMP6)
    {
        ns_hdr->chksum = ip6_chksum_pseudo(p, IP6_NEXTH_ICMP6, p->len, src_addr,
                                           target_addr);
    }
#endif /* CHECKSUM_GEN_ICMP6 */

    /* Send the packet out. */
    ND6_STATS_INC(nd6.xmit);
    ip6_output_if(p, (src_addr == IP6_ADDR_ANY6) ? NULL : src_addr, target_addr,
                  LWIP_ICMP6_HL, 0, IP6_NEXTH_ICMP6, netif);
    pbuf_free(p);
}

/**
 * Send a neighbor advertisement message
 *
 * @param netif the netif on which to send the message
 * @param target_addr the IPv6 target address for the ND message
 * @param flags one of ND6_SEND_FLAG_*
 */
static void
nd6_send_na(struct netif *netif, const ip6_addr_t *target_addr, u8_t flags)
{
    struct na_header *na_hdr;
    struct lladdr_option *lladdr_opt;
    struct pbuf *p;
    const ip6_addr_t *src_addr;
    const ip6_addr_t *dest_addr;
    u16_t lladdr_opt_len;

    /* Use link-local address as source address. */
    /* src_addr = netif_ip6_addr(netif, 0); */
    /* Use target address as source address. */
    src_addr = target_addr;

    /* Allocate a packet. */
    lladdr_opt_len = ((netif->hwaddr_len + 2) >> 3) + (((netif->hwaddr_len + 2) & 0x07) ? 1 : 0);
    p = pbuf_alloc(PBUF_IP, sizeof(struct na_header) + (lladdr_opt_len << 3), PBUF_RAM);
    if (p == NULL)
    {
        ND6_STATS_INC(nd6.memerr);
        return;
    }

    /* Set fields. */
    na_hdr = (struct na_header *)p->payload;
    lladdr_opt = (struct lladdr_option *)((u8_t *)p->payload + sizeof(struct na_header));

    na_hdr->type = ICMP6_TYPE_NA;
    na_hdr->code = 0;
    na_hdr->chksum = 0;
    na_hdr->flags = flags & 0xf0;
    na_hdr->reserved[0] = 0;
    na_hdr->reserved[1] = 0;
    na_hdr->reserved[2] = 0;
    ip6_addr_set(&(na_hdr->target_address), target_addr);

    lladdr_opt->type = ND6_OPTION_TYPE_TARGET_LLADDR;
    lladdr_opt->length = (u8_t)lladdr_opt_len;
    SMEMCPY(lladdr_opt->addr, netif->hwaddr, netif->hwaddr_len);

    /* Generate the solicited node address for the target address. */
    if (flags & ND6_SEND_FLAG_MULTICAST_DEST)
    {
        ip6_addr_set_solicitednode(&multicast_address, target_addr->addr[3]);
        dest_addr = &multicast_address;
    }
    else if (flags & ND6_SEND_FLAG_ALLNODES_DEST)
    {
        ip6_addr_set_allnodes_linklocal(&multicast_address);
        dest_addr = &multicast_address;
    }
    else
    {
        dest_addr = ip6_current_src_addr();
    }

#if CHECKSUM_GEN_ICMP6
    IF__NETIF_CHECKSUM_ENABLED(netif, NETIF_CHECKSUM_GEN_ICMP6)
    {
        na_hdr->chksum = ip6_chksum_pseudo(p, IP6_NEXTH_ICMP6, p->len, src_addr,
                                           dest_addr);
    }
#endif /* CHECKSUM_GEN_ICMP6 */

    /* Send the packet out. */
    ND6_STATS_INC(nd6.xmit);
    ip6_output_if(p, src_addr, dest_addr,
                  LWIP_ICMP6_HL, 0, IP6_NEXTH_ICMP6, netif);
    pbuf_free(p);
}

#if LWIP_IPV6_SEND_ROUTER_SOLICIT
/**
 * Send a router solicitation message
 *
 * @param netif the netif on which to send the message
 */
static err_t
nd6_send_rs(struct netif *netif)
{
    struct rs_header *rs_hdr;
    struct lladdr_option *lladdr_opt;
    struct pbuf *p;
    const ip6_addr_t *src_addr;
    err_t err;
    u16_t lladdr_opt_len = 0;

    /* Link-local source address, or unspecified address? */
    if (ip6_addr_isvalid(netif_ip6_addr_state(netif, 0)))
    {
        src_addr = netif_ip6_addr(netif, 0);
    }
    else
    {
        src_addr = IP6_ADDR_ANY6;
    }

    /* Generate the all routers target address. */
    ip6_addr_set_allrouters_linklocal(&multicast_address);

    /* Allocate a packet. */
    if (src_addr != IP6_ADDR_ANY6)
    {
        lladdr_opt_len = ((netif->hwaddr_len + 2) >> 3) + (((netif->hwaddr_len + 2) & 0x07) ? 1 : 0);
    }
    p = pbuf_alloc(PBUF_IP, sizeof(struct rs_header) + (lladdr_opt_len << 3), PBUF_RAM);
    if (p == NULL)
    {
        ND6_STATS_INC(nd6.memerr);
        return ERR_BUF;
    }

    /* Set fields. */
    rs_hdr = (struct rs_header *)p->payload;

    rs_hdr->type = ICMP6_TYPE_RS;
    rs_hdr->code = 0;
    rs_hdr->chksum = 0;
    rs_hdr->reserved = 0;

    if (src_addr != IP6_ADDR_ANY6)
    {
        /* Include our hw address. */
        lladdr_opt = (struct lladdr_option *)((u8_t *)p->payload + sizeof(struct rs_header));
        lladdr_opt->type = ND6_OPTION_TYPE_SOURCE_LLADDR;
        lladdr_opt->length = (u8_t)lladdr_opt_len;
        SMEMCPY(lladdr_opt->addr, netif->hwaddr, netif->hwaddr_len);
    }

#if CHECKSUM_GEN_ICMP6
    IF__NETIF_CHECKSUM_ENABLED(netif, NETIF_CHECKSUM_GEN_ICMP6)
    {
        rs_hdr->chksum = ip6_chksum_pseudo(p, IP6_NEXTH_ICMP6, p->len, src_addr,
                                           &multicast_address);
    }
#endif /* CHECKSUM_GEN_ICMP6 */

    /* Send the packet out. */
    ND6_STATS_INC(nd6.xmit);

    err = ip6_output_if(p, (src_addr == IP6_ADDR_ANY6) ? NULL : src_addr, &multicast_address,
                        LWIP_ICMP6_HL, 0, IP6_NEXTH_ICMP6, netif);
    pbuf_free(p);

    return err;
}
#endif /* LWIP_IPV6_SEND_ROUTER_SOLICIT */

/**
 * Search for a neighbor cache entry
 *
 * @param ip6addr the IPv6 address of the neighbor
 * @return The neighbor cache entry index that matched, -1 if no
 * entry is found
 */
static s8_t
nd6_find_neighbor_cache_entry(const ip6_addr_t *ip6addr)
{
    s8_t i;
    for (i = 0; i < LWIP_ND6_NUM_NEIGHBORS; i++)
    {
        if (ip6_addr_cmp(ip6addr, &(neighbor_cache[i].next_hop_address)))
        {
            return i;
        }
    }
    return -1;
}

/**
 * Create a new neighbor cache entry.
 *
 * If no unused entry is found, will try to recycle an old entry
 * according to ad-hoc "age" heuristic.
 *
 * @return The neighbor cache entry index that was created, -1 if no
 * entry could be created
 */
static s8_t
nd6_new_neighbor_cache_entry(void)
{
    s8_t i;
    s8_t j;
    u32_t time;


    /* First, try to find an empty entry. */
    for (i = 0; i < LWIP_ND6_NUM_NEIGHBORS; i++)
    {
        if (neighbor_cache[i].state == ND6_NO_ENTRY)
        {
            return i;
        }
    }

    /* We need to recycle an entry. in general, do not recycle if it is a router. */

    /* Next, try to find a Stale entry. */
    for (i = 0; i < LWIP_ND6_NUM_NEIGHBORS; i++)
    {
        if ((neighbor_cache[i].state == ND6_STALE) &&
                (!neighbor_cache[i].isrouter))
        {
            nd6_free_neighbor_cache_entry(i);
            return i;
        }
    }

    /* Next, try to find a Probe entry. */
    for (i = 0; i < LWIP_ND6_NUM_NEIGHBORS; i++)
    {
        if ((neighbor_cache[i].state == ND6_PROBE) &&
                (!neighbor_cache[i].isrouter))
        {
            nd6_free_neighbor_cache_entry(i);
            return i;
        }
    }

    /* Next, try to find a Delayed entry. */
    for (i = 0; i < LWIP_ND6_NUM_NEIGHBORS; i++)
    {
        if ((neighbor_cache[i].state == ND6_DELAY) &&
                (!neighbor_cache[i].isrouter))
        {
            nd6_free_neighbor_cache_entry(i);
            return i;
        }
    }

    /* Next, try to find the oldest reachable entry. */
    time = 0xfffffffful;
    j = -1;
    for (i = 0; i < LWIP_ND6_NUM_NEIGHBORS; i++)
    {
        if ((neighbor_cache[i].state == ND6_REACHABLE) &&
                (!neighbor_cache[i].isrouter))
        {
            if (neighbor_cache[i].counter.reachable_time < time)
            {
                j = i;
                time = neighbor_cache[i].counter.reachable_time;
            }
        }
    }
    if (j >= 0)
    {
        nd6_free_neighbor_cache_entry(j);
        return j;
    }

    /* Next, find oldest incomplete entry without queued packets. */
    time = 0;
    j = -1;
    for (i = 0; i < LWIP_ND6_NUM_NEIGHBORS; i++)
    {
        if (
            (neighbor_cache[i].q == NULL) &&
            (neighbor_cache[i].state == ND6_INCOMPLETE) &&
            (!neighbor_cache[i].isrouter))
        {
            if (neighbor_cache[i].counter.probes_sent >= time)
            {
                j = i;
                time = neighbor_cache[i].counter.probes_sent;
            }
        }
    }
    if (j >= 0)
    {
        nd6_free_neighbor_cache_entry(j);
        return j;
    }

    /* Next, find oldest incomplete entry with queued packets. */
    time = 0;
    j = -1;
    for (i = 0; i < LWIP_ND6_NUM_NEIGHBORS; i++)
    {
        if ((neighbor_cache[i].state == ND6_INCOMPLETE) &&
                (!neighbor_cache[i].isrouter))
        {
            if (neighbor_cache[i].counter.probes_sent >= time)
            {
                j = i;
                time = neighbor_cache[i].counter.probes_sent;
            }
        }
    }
    if (j >= 0)
    {
        nd6_free_neighbor_cache_entry(j);
        return j;
    }

    /* No more entries to try. */
    return -1;
}

/**
 * Will free any resources associated with a neighbor cache
 * entry, and will mark it as unused.
 *
 * @param i the neighbor cache entry index to free
 */
static void
nd6_free_neighbor_cache_entry(s8_t i)
{
    if ((i < 0) || (i >= LWIP_ND6_NUM_NEIGHBORS))
    {
        return;
    }
    if (neighbor_cache[i].isrouter)
    {
        /* isrouter needs to be cleared before deleting a neighbor cache entry */
        return;
    }

    /* Free any queued packets. */
    if (neighbor_cache[i].q != NULL)
    {
        nd6_free_q(neighbor_cache[i].q);
        neighbor_cache[i].q = NULL;
    }

    neighbor_cache[i].state = ND6_NO_ENTRY;
    neighbor_cache[i].isrouter = 0;
    neighbor_cache[i].netif = NULL;
    neighbor_cache[i].counter.reachable_time = 0;
    ip6_addr_set_zero(&(neighbor_cache[i].next_hop_address));
}

/**
 * Search for a destination cache entry
 *
 * @param ip6addr the IPv6 address of the destination
 * @return The destination cache entry index that matched, -1 if no
 * entry is found
 */
static s8_t
nd6_find_destination_cache_entry(const ip6_addr_t *ip6addr)
{
    s8_t i;
    for (i = 0; i < LWIP_ND6_NUM_DESTINATIONS; i++)
    {
        if (ip6_addr_cmp(ip6addr, &(destination_cache[i].destination_addr)))
        {
            return i;
        }
    }
    return -1;
}

/**
 * Create a new destination cache entry. If no unused entry is found,
 * will recycle oldest entry.
 *
 * @return The destination cache entry index that was created, -1 if no
 * entry was created
 */
static s8_t
nd6_new_destination_cache_entry(void)
{
    s8_t i, j;
    u32_t age;

    /* Find an empty entry. */
    for (i = 0; i < LWIP_ND6_NUM_DESTINATIONS; i++)
    {
        if (ip6_addr_isany(&(destination_cache[i].destination_addr)))
        {
            return i;
        }
    }

    /* Find oldest entry. */
    age = 0;
    j = LWIP_ND6_NUM_DESTINATIONS - 1;
    for (i = 0; i < LWIP_ND6_NUM_DESTINATIONS; i++)
    {
        if (destination_cache[i].age > age)
        {
            j = i;
        }
    }

    return j;
}

/**
 * Clear the destination cache.
 *
 * This operation may be necessary for consistency in the light of changing
 * local addresses and/or use of the gateway hook.
 */
void
nd6_clear_destination_cache(void)
{
    int i;

    for (i = 0; i < LWIP_ND6_NUM_DESTINATIONS; i++)
    {
        ip6_addr_set_any(&destination_cache[i].destination_addr);
    }
}

/**
 * Determine whether an address matches an on-link prefix.
 *
 * @param ip6addr the IPv6 address to match
 * @return 1 if the address is on-link, 0 otherwise
 */
static s8_t
nd6_is_prefix_in_netif(const ip6_addr_t *ip6addr, struct netif *netif)
{
    s8_t i;
    for (i = 0; i < LWIP_ND6_NUM_PREFIXES; i++)
    {
        if ((prefix_list[i].netif == netif) &&
                (prefix_list[i].invalidation_timer > 0) &&
                ip6_addr_netcmp(ip6addr, &(prefix_list[i].prefix)))
        {
            return 1;
        }
    }
    /* Check to see if address prefix matches a (manually?) configured address. */
    for (i = 0; i < LWIP_IPV6_NUM_ADDRESSES; i++)
    {
        if (ip6_addr_isvalid(netif_ip6_addr_state(netif, i)) &&
                ip6_addr_netcmp(ip6addr, netif_ip6_addr(netif, i)))
        {
            return 1;
        }
    }
    return 0;
}

/**
 * Select a default router for a destination.
 *
 * @param ip6addr the destination address
 * @param netif the netif for the outgoing packet, if known
 * @return the default router entry index, or -1 if no suitable
 *         router is found
 */
static s8_t
nd6_select_router(const ip6_addr_t *ip6addr, struct netif *netif)
{
    s8_t i;
    /* last_router is used for round-robin router selection (as recommended
     * in RFC). This is more robust in case one router is not reachable,
     * we are not stuck trying to resolve it. */
    static s8_t last_router;
    (void)ip6addr; /* @todo match preferred routes!! (must implement ND6_OPTION_TYPE_ROUTE_INFO) */

    /* @todo: implement default router preference */

    /* Look for reachable routers. */
    for (i = 0; i < LWIP_ND6_NUM_ROUTERS; i++)
    {
        if (++last_router >= LWIP_ND6_NUM_ROUTERS)
        {
            last_router = 0;
        }
        if ((default_router_list[i].neighbor_entry != NULL) &&
                (netif != NULL ? netif == default_router_list[i].neighbor_entry->netif : 1) &&
                (default_router_list[i].invalidation_timer > 0) &&
                (default_router_list[i].neighbor_entry->state == ND6_REACHABLE))
        {
            return i;
        }
    }

    /* Look for router in other reachability states, but still valid according to timer. */
    for (i = 0; i < LWIP_ND6_NUM_ROUTERS; i++)
    {
        if (++last_router >= LWIP_ND6_NUM_ROUTERS)
        {
            last_router = 0;
        }
        if ((default_router_list[i].neighbor_entry != NULL) &&
                (netif != NULL ? netif == default_router_list[i].neighbor_entry->netif : 1) &&
                (default_router_list[i].invalidation_timer > 0))
        {
            return i;
        }
    }

    /* Look for any router for which we have any information at all. */
    for (i = 0; i < LWIP_ND6_NUM_ROUTERS; i++)
    {
        if (++last_router >= LWIP_ND6_NUM_ROUTERS)
        {
            last_router = 0;
        }
        if (default_router_list[i].neighbor_entry != NULL &&
                (netif != NULL ? netif == default_router_list[i].neighbor_entry->netif : 1))
        {
            return i;
        }
    }

    /* no suitable router found. */
    return -1;
}

/**
 * Find a router-announced route to the given destination.
 *
 * The caller is responsible for checking whether the returned netif, if any,
 * is in a suitable state (up, link up) to be used for packet transmission.
 *
 * @param ip6addr the destination IPv6 address
 * @return the netif to use for the destination, or NULL if none found
 */
struct netif *
nd6_find_route(const ip6_addr_t *ip6addr)
{
    s8_t i;

    i = nd6_select_router(ip6addr, NULL);
    if (i >= 0)
    {
        if (default_router_list[i].neighbor_entry != NULL)
        {
            return default_router_list[i].neighbor_entry->netif; /* may be NULL */
        }
    }

    return NULL;
}

/**
 * Find an entry for a default router.
 *
 * @param router_addr the IPv6 address of the router
 * @param netif the netif on which the router is found, if known
 * @return the index of the router entry, or -1 if not found
 */
static s8_t
nd6_get_router(const ip6_addr_t *router_addr, struct netif *netif)
{
    s8_t i;

    /* Look for router. */
    for (i = 0; i < LWIP_ND6_NUM_ROUTERS; i++)
    {
        if ((default_router_list[i].neighbor_entry != NULL) &&
                ((netif != NULL) ? netif == default_router_list[i].neighbor_entry->netif : 1) &&
                ip6_addr_cmp(router_addr, &(default_router_list[i].neighbor_entry->next_hop_address)))
        {
            return i;
        }
    }

    /* router not found. */
    return -1;
}

/**
 * Create a new entry for a default router.
 *
 * @param router_addr the IPv6 address of the router
 * @param netif the netif on which the router is connected, if known
 * @return the index on the router table, or -1 if could not be created
 */
static s8_t
nd6_new_router(const ip6_addr_t *router_addr, struct netif *netif)
{
    s8_t router_index;
    s8_t free_router_index;
    s8_t neighbor_index;

    /* Do we have a neighbor entry for this router? */
    neighbor_index = nd6_find_neighbor_cache_entry(router_addr);
    if (neighbor_index < 0)
    {
        /* Create a neighbor entry for this router. */
        neighbor_index = nd6_new_neighbor_cache_entry();
        if (neighbor_index < 0)
        {
            /* Could not create neighbor entry for this router. */
            return -1;
        }
        ip6_addr_set(&(neighbor_cache[neighbor_index].next_hop_address), router_addr);
        neighbor_cache[neighbor_index].netif = netif;
        neighbor_cache[neighbor_index].q = NULL;
        neighbor_cache[neighbor_index].state = ND6_INCOMPLETE;
        neighbor_cache[neighbor_index].counter.probes_sent = 1;
        nd6_send_neighbor_cache_probe(&neighbor_cache[neighbor_index], ND6_SEND_FLAG_MULTICAST_DEST);
    }

    /* Mark neighbor as router. */
    neighbor_cache[neighbor_index].isrouter = 1;

    /* Look for empty entry. */
    free_router_index = LWIP_ND6_NUM_ROUTERS;
    for (router_index = LWIP_ND6_NUM_ROUTERS - 1; router_index >= 0; router_index--)
    {
        /* check if router already exists (this is a special case for 2 netifs on the same subnet
           - e.g. wifi and cable) */
        if(default_router_list[router_index].neighbor_entry == &(neighbor_cache[neighbor_index]))
        {
            return router_index;
        }
        if (default_router_list[router_index].neighbor_entry == NULL)
        {
            /* remember lowest free index to create a new entry */
            free_router_index = router_index;
        }
    }
    if (free_router_index < LWIP_ND6_NUM_ROUTERS)
    {
        default_router_list[free_router_index].neighbor_entry = &(neighbor_cache[neighbor_index]);
        return free_router_index;
    }

    /* Could not create a router entry. */

    /* Mark neighbor entry as not-router. Entry might be useful as neighbor still. */
    neighbor_cache[neighbor_index].isrouter = 0;

    /* router not found. */
    return -1;
}

/**
 * Find the cached entry for an on-link prefix.
 *
 * @param prefix the IPv6 prefix that is on-link
 * @param netif the netif on which the prefix is on-link
 * @return the index on the prefix table, or -1 if not found
 */
static s8_t
nd6_get_onlink_prefix(ip6_addr_t *prefix, struct netif *netif)
{
    s8_t i;

    /* Look for prefix in list. */
    for (i = 0; i < LWIP_ND6_NUM_PREFIXES; ++i)
    {
        if ((ip6_addr_netcmp(&(prefix_list[i].prefix), prefix)) &&
                (prefix_list[i].netif == netif))
        {
            return i;
        }
    }

    /* Entry not available. */
    return -1;
}

/**
 * Creates a new entry for an on-link prefix.
 *
 * @param prefix the IPv6 prefix that is on-link
 * @param netif the netif on which the prefix is on-link
 * @return the index on the prefix table, or -1 if not created
 */
static s8_t
nd6_new_onlink_prefix(ip6_addr_t *prefix, struct netif *netif)
{
    s8_t i;

    /* Create new entry. */
    for (i = 0; i < LWIP_ND6_NUM_PREFIXES; ++i)
    {
        if ((prefix_list[i].netif == NULL) ||
                (prefix_list[i].invalidation_timer == 0))
        {
            /* Found empty prefix entry. */
            prefix_list[i].netif = netif;
            ip6_addr_set(&(prefix_list[i].prefix), prefix);
#if LWIP_IPV6_AUTOCONFIG
            prefix_list[i].flags = 0;
#endif /* LWIP_IPV6_AUTOCONFIG */
            return i;
        }
    }

    /* Entry not available. */
    return -1;
}

/**
 * Determine the next hop for a destination. Will determine if the
 * destination is on-link, else a suitable on-link router is selected.
 *
 * The last entry index is cached for fast entry search.
 *
 * @param ip6addr the destination address
 * @param netif the netif on which the packet will be sent
 * @return the neighbor cache entry for the next hop, ERR_RTE if no
 *         suitable next hop was found, ERR_MEM if no cache entry
 *         could be created
 */
static s8_t
nd6_get_next_hop_entry(const ip6_addr_t *ip6addr, struct netif *netif)
{
#ifdef LWIP_HOOK_ND6_GET_GW
    const ip6_addr_t *next_hop_addr;
#endif /* LWIP_HOOK_ND6_GET_GW */
    s8_t i;

#if LWIP_NETIF_HWADDRHINT
    if (netif->addr_hint != NULL)
    {
        /* per-pcb cached entry was given */
        u8_t addr_hint = *(netif->addr_hint);
        if (addr_hint < LWIP_ND6_NUM_DESTINATIONS)
        {
            nd6_cached_destination_index = addr_hint;
        }
    }
#endif /* LWIP_NETIF_HWADDRHINT */

    /* Look for ip6addr in destination cache. */
    if (ip6_addr_cmp(ip6addr, &(destination_cache[nd6_cached_destination_index].destination_addr)))
    {
        /* the cached entry index is the right one! */
        /* do nothing. */
        ND6_STATS_INC(nd6.cachehit);
    }
    else
    {
        /* Search destination cache. */
        i = nd6_find_destination_cache_entry(ip6addr);
        if (i >= 0)
        {
            /* found destination entry. make it our new cached index. */
            nd6_cached_destination_index = i;
        }
        else
        {
            /* Not found. Create a new destination entry. */
            i = nd6_new_destination_cache_entry();
            if (i >= 0)
            {
                /* got new destination entry. make it our new cached index. */
                nd6_cached_destination_index = i;
            }
            else
            {
                /* Could not create a destination cache entry. */
                return ERR_MEM;
            }

            /* Copy dest address to destination cache. */
            ip6_addr_set(&(destination_cache[nd6_cached_destination_index].destination_addr), ip6addr);

            /* Now find the next hop. is it a neighbor? */
            if (ip6_addr_islinklocal(ip6addr) ||
                    nd6_is_prefix_in_netif(ip6addr, netif))
            {
                /* Destination in local link. */
                destination_cache[nd6_cached_destination_index].pmtu = netif->mtu;
                ip6_addr_copy(destination_cache[nd6_cached_destination_index].next_hop_addr, destination_cache[nd6_cached_destination_index].destination_addr);
#ifdef LWIP_HOOK_ND6_GET_GW
            }
            else if ((next_hop_addr = LWIP_HOOK_ND6_GET_GW(netif, ip6addr)) != NULL)
            {
                /* Next hop for destination provided by hook function. */
                destination_cache[nd6_cached_destination_index].pmtu = netif->mtu;
                ip6_addr_set(&destination_cache[nd6_cached_destination_index].next_hop_addr, next_hop_addr);
#endif /* LWIP_HOOK_ND6_GET_GW */
            }
            else
            {
                /* We need to select a router. */
                i = nd6_select_router(ip6addr, netif);
                if (i < 0)
                {
                    /* No router found. */
                    ip6_addr_set_any(&(destination_cache[nd6_cached_destination_index].destination_addr));
                    return ERR_RTE;
                }
                destination_cache[nd6_cached_destination_index].pmtu = netif->mtu; /* Start with netif mtu, correct through ICMPv6 if necessary */
                ip6_addr_copy(destination_cache[nd6_cached_destination_index].next_hop_addr, default_router_list[i].neighbor_entry->next_hop_address);
            }
        }
    }

#if LWIP_NETIF_HWADDRHINT
    if (netif->addr_hint != NULL)
    {
        /* per-pcb cached entry was given */
        *(netif->addr_hint) = nd6_cached_destination_index;
    }
#endif /* LWIP_NETIF_HWADDRHINT */

    /* Look in neighbor cache for the next-hop address. */
    if (ip6_addr_cmp(&(destination_cache[nd6_cached_destination_index].next_hop_addr),
                     &(neighbor_cache[nd6_cached_neighbor_index].next_hop_address)))
    {
        /* Cache hit. */
        /* Do nothing. */
        ND6_STATS_INC(nd6.cachehit);
    }
    else
    {
        i = nd6_find_neighbor_cache_entry(&(destination_cache[nd6_cached_destination_index].next_hop_addr));
        if (i >= 0)
        {
            /* Found a matching record, make it new cached entry. */
            nd6_cached_neighbor_index = i;
        }
        else
        {
            /* Neighbor not in cache. Make a new entry. */
            i = nd6_new_neighbor_cache_entry();
            if (i >= 0)
            {
                /* got new neighbor entry. make it our new cached index. */
                nd6_cached_neighbor_index = i;
            }
            else
            {
                /* Could not create a neighbor cache entry. */
                return ERR_MEM;
            }

            /* Initialize fields. */
            ip6_addr_copy(neighbor_cache[i].next_hop_address,
                          destination_cache[nd6_cached_destination_index].next_hop_addr);
            neighbor_cache[i].isrouter = 0;
            neighbor_cache[i].netif = netif;
            neighbor_cache[i].state = ND6_INCOMPLETE;
            neighbor_cache[i].counter.probes_sent = 1;
            nd6_send_neighbor_cache_probe(&neighbor_cache[i], ND6_SEND_FLAG_MULTICAST_DEST);
        }
    }

    /* Reset this destination's age. */
    destination_cache[nd6_cached_destination_index].age = 0;

    return nd6_cached_neighbor_index;
}

/**
 * Queue a packet for a neighbor.
 *
 * @param neighbor_index the index in the neighbor cache table
 * @param q packet to be queued
 * @return ERR_OK if succeeded, ERR_MEM if out of memory
 */
static err_t
nd6_queue_packet(s8_t neighbor_index, struct pbuf *q)
{
    err_t result = ERR_MEM;
    struct pbuf *p;
    int copy_needed = 0;
#if LWIP_ND6_QUEUEING
    struct nd6_q_entry *new_entry, *r;
#endif /* LWIP_ND6_QUEUEING */

    if ((neighbor_index < 0) || (neighbor_index >= LWIP_ND6_NUM_NEIGHBORS))
    {
        return ERR_ARG;
    }

    /* IF q includes a PBUF_REF, PBUF_POOL or PBUF_RAM, we have no choice but
     * to copy the whole queue into a new PBUF_RAM (see bug #11400)
     * PBUF_ROMs can be left as they are, since ROM must not get changed. */
    p = q;
    while (p)
    {
        if (p->type != PBUF_ROM)
        {
            copy_needed = 1;
            break;
        }
        p = p->next;
    }
    if (copy_needed)
    {
        /* copy the whole packet into new pbufs */
        p = pbuf_alloc(PBUF_LINK, q->tot_len, PBUF_RAM);
        while ((p == NULL) && (neighbor_cache[neighbor_index].q != NULL))
        {
            /* Free oldest packet (as per RFC recommendation) */
#if LWIP_ND6_QUEUEING
            r = neighbor_cache[neighbor_index].q;
            neighbor_cache[neighbor_index].q = r->next;
            r->next = NULL;
            nd6_free_q(r);
#else /* LWIP_ND6_QUEUEING */
            pbuf_free(neighbor_cache[neighbor_index].q);
            neighbor_cache[neighbor_index].q = NULL;
#endif /* LWIP_ND6_QUEUEING */
            p = pbuf_alloc(PBUF_LINK, q->tot_len, PBUF_RAM);
        }
        if (p != NULL)
        {
            if (pbuf_copy(p, q) != ERR_OK)
            {
                pbuf_free(p);
                p = NULL;
            }
        }
    }
    else
    {
        /* referencing the old pbuf is enough */
        p = q;
        pbuf_ref(p);
    }
    /* packet was copied/ref'd? */
    if (p != NULL)
    {
        /* queue packet ... */
#if LWIP_ND6_QUEUEING
        /* allocate a new nd6 queue entry */
        new_entry = (struct nd6_q_entry *)memp_malloc(MEMP_ND6_QUEUE);
        if ((new_entry == NULL) && (neighbor_cache[neighbor_index].q != NULL))
        {
            /* Free oldest packet (as per RFC recommendation) */
            r = neighbor_cache[neighbor_index].q;
            neighbor_cache[neighbor_index].q = r->next;
            r->next = NULL;
            nd6_free_q(r);
            new_entry = (struct nd6_q_entry *)memp_malloc(MEMP_ND6_QUEUE);
        }
        if (new_entry != NULL)
        {
            new_entry->next = NULL;
            new_entry->p = p;
            if (neighbor_cache[neighbor_index].q != NULL)
            {
                /* queue was already existent, append the new entry to the end */
                r = neighbor_cache[neighbor_index].q;
                while (r->next != NULL)
                {
                    r = r->next;
                }
                r->next = new_entry;
            }
            else
            {
                /* queue did not exist, first item in queue */
                neighbor_cache[neighbor_index].q = new_entry;
            }
            LWIP_DEBUGF(LWIP_DBG_TRACE, ("ipv6: queued packet %p on neighbor entry %"S16_F"\n", (void *)p, (s16_t)neighbor_index));
            result = ERR_OK;
        }
        else
        {
            /* the pool MEMP_ND6_QUEUE is empty */
            pbuf_free(p);
            LWIP_DEBUGF(LWIP_DBG_TRACE, ("ipv6: could not queue a copy of packet %p (out of memory)\n", (void *)p));
            /* { result == ERR_MEM } through initialization */
        }
#else /* LWIP_ND6_QUEUEING */
        /* Queue a single packet. If an older packet is already queued, free it as per RFC. */
        if (neighbor_cache[neighbor_index].q != NULL)
        {
            pbuf_free(neighbor_cache[neighbor_index].q);
        }
        neighbor_cache[neighbor_index].q = p;
        LWIP_DEBUGF(LWIP_DBG_TRACE, ("ipv6: queued packet %p on neighbor entry %"S16_F"\n", (void *)p, (s16_t)neighbor_index));
        result = ERR_OK;
#endif /* LWIP_ND6_QUEUEING */
    }
    else
    {
        LWIP_DEBUGF(LWIP_DBG_TRACE, ("ipv6: could not queue a copy of packet %p (out of memory)\n", (void *)q));
        /* { result == ERR_MEM } through initialization */
    }

    return result;
}

#if LWIP_ND6_QUEUEING
/**
 * Free a complete queue of nd6 q entries
 *
 * @param q a queue of nd6_q_entry to free
 */
static void
nd6_free_q(struct nd6_q_entry *q)
{
    struct nd6_q_entry *r;
    LWIP_ASSERT("q != NULL", q != NULL);
    LWIP_ASSERT("q->p != NULL", q->p != NULL);
    while (q)
    {
        r = q;
        q = q->next;
        LWIP_ASSERT("r->p != NULL", (r->p != NULL));
        pbuf_free(r->p);
        memp_free(MEMP_ND6_QUEUE, r);
    }
}
#endif /* LWIP_ND6_QUEUEING */

/**
 * Send queued packets for a neighbor
 *
 * @param i the neighbor to send packets to
 */
static void
nd6_send_q(s8_t i)
{
    struct ip6_hdr *ip6hdr;
    ip6_addr_t dest;
#if LWIP_ND6_QUEUEING
    struct nd6_q_entry *q;
#endif /* LWIP_ND6_QUEUEING */

    if ((i < 0) || (i >= LWIP_ND6_NUM_NEIGHBORS))
    {
        return;
    }

#if LWIP_ND6_QUEUEING
    while (neighbor_cache[i].q != NULL)
    {
        /* remember first in queue */
        q = neighbor_cache[i].q;
        /* pop first item off the queue */
        neighbor_cache[i].q = q->next;
        /* Get ipv6 header. */
        ip6hdr = (struct ip6_hdr *)(q->p->payload);
        /* Create an aligned copy. */
        ip6_addr_set(&dest, &(ip6hdr->dest));
        /* send the queued IPv6 packet */
        (neighbor_cache[i].netif)->output_ip6(neighbor_cache[i].netif, q->p, &dest);
        /* free the queued IP packet */
        pbuf_free(q->p);
        /* now queue entry can be freed */
        memp_free(MEMP_ND6_QUEUE, q);
    }
#else /* LWIP_ND6_QUEUEING */
    if (neighbor_cache[i].q != NULL)
    {
        /* Get ipv6 header. */
        ip6hdr = (struct ip6_hdr *)(neighbor_cache[i].q->payload);
        /* Create an aligned copy. */
        ip6_addr_set(&dest, &(ip6hdr->dest));
        /* send the queued IPv6 packet */
        (neighbor_cache[i].netif)->output_ip6(neighbor_cache[i].netif, neighbor_cache[i].q, &dest);
        /* free the queued IP packet */
        pbuf_free(neighbor_cache[i].q);
        neighbor_cache[i].q = NULL;
    }
#endif /* LWIP_ND6_QUEUEING */
}

/**
 * A packet is to be transmitted to a specific IPv6 destination on a specific
 * interface. Check if we can find the hardware address of the next hop to use
 * for the packet. If so, give the hardware address to the caller, which should
 * use it to send the packet right away. Otherwise, enqueue the packet for
 * later transmission while looking up the hardware address, if possible.
 *
 * As such, this function returns one of three different possible results:
 *
 * - ERR_OK with a non-NULL 'hwaddrp': the caller should send the packet now.
 * - ERR_OK with a NULL 'hwaddrp': the packet has been enqueued for later.
 * - not ERR_OK: something went wrong; forward the error upward in the stack.
 *
 * @param netif The lwIP network interface on which the IP packet will be sent.
 * @param q The pbuf(s) containing the IP packet to be sent.
 * @param ip6addr The destination IPv6 address of the packet.
 * @param hwaddrp On success, filled with a pointer to a HW address or NULL (meaning
 *        the packet has been queued).
 * @return
 * - ERR_OK on success, ERR_RTE if no route was found for the packet,
 * or ERR_MEM if low memory conditions prohibit sending the packet at all.
 */
err_t
nd6_get_next_hop_addr_or_queue(struct netif *netif, struct pbuf *q, const ip6_addr_t *ip6addr, const u8_t **hwaddrp)
{
    s8_t i;

    /* Get next hop record. */
    i = nd6_get_next_hop_entry(ip6addr, netif);
    if (i < 0)
    {
        /* failed to get a next hop neighbor record. */
        return i;
    }

    /* Now that we have a destination record, send or queue the packet. */
    if (neighbor_cache[i].state == ND6_STALE)
    {
        /* Switch to delay state. */
        neighbor_cache[i].state = ND6_DELAY;
        neighbor_cache[i].counter.delay_time = LWIP_ND6_DELAY_FIRST_PROBE_TIME / ND6_TMR_INTERVAL;
    }
    /* @todo should we send or queue if PROBE? send for now, to let unicast NS pass. */
    if ((neighbor_cache[i].state == ND6_REACHABLE) ||
            (neighbor_cache[i].state == ND6_DELAY) ||
            (neighbor_cache[i].state == ND6_PROBE))
    {

        /* Tell the caller to send out the packet now. */
        *hwaddrp = neighbor_cache[i].lladdr;
        return ERR_OK;
    }

    /* We should queue packet on this interface. */
    *hwaddrp = NULL;
    return nd6_queue_packet(i, q);
}


/**
 * Get the Path MTU for a destination.
 *
 * @param ip6addr the destination address
 * @param netif the netif on which the packet will be sent
 * @return the Path MTU, if known, or the netif default MTU
 */
u16_t
nd6_get_destination_mtu(const ip6_addr_t *ip6addr, struct netif *netif)
{
    s8_t i;

    i = nd6_find_destination_cache_entry(ip6addr);
    if (i >= 0)
    {
        if (destination_cache[i].pmtu > 0)
        {
            return destination_cache[i].pmtu;
        }
    }

    if (netif != NULL)
    {
        return netif->mtu;
    }

    return 1280; /* Minimum MTU */
}


#if LWIP_ND6_TCP_REACHABILITY_HINTS
/**
 * Provide the Neighbor discovery process with a hint that a
 * destination is reachable. Called by tcp_receive when ACKs are
 * received or sent (as per RFC). This is useful to avoid sending
 * NS messages every 30 seconds.
 *
 * @param ip6addr the destination address which is know to be reachable
 *                by an upper layer protocol (TCP)
 */
void
nd6_reachability_hint(const ip6_addr_t *ip6addr)
{
    s8_t i;

    /* Find destination in cache. */
    if (ip6_addr_cmp(ip6addr, &(destination_cache[nd6_cached_destination_index].destination_addr)))
    {
        i = nd6_cached_destination_index;
        ND6_STATS_INC(nd6.cachehit);
    }
    else
    {
        i = nd6_find_destination_cache_entry(ip6addr);
    }
    if (i < 0)
    {
        return;
    }

    /* Find next hop neighbor in cache. */
    if (ip6_addr_cmp(&(destination_cache[i].next_hop_addr), &(neighbor_cache[nd6_cached_neighbor_index].next_hop_address)))
    {
        i = nd6_cached_neighbor_index;
        ND6_STATS_INC(nd6.cachehit);
    }
    else
    {
        i = nd6_find_neighbor_cache_entry(&(destination_cache[i].next_hop_addr));
    }
    if (i < 0)
    {
        return;
    }

    /* For safety: don't set as reachable if we don't have a LL address yet. Misuse protection. */
    if (neighbor_cache[i].state == ND6_INCOMPLETE || neighbor_cache[i].state == ND6_NO_ENTRY)
    {
        return;
    }

    /* Set reachability state. */
    neighbor_cache[i].state = ND6_REACHABLE;
    neighbor_cache[i].counter.reachable_time = reachable_time;
}
#endif /* LWIP_ND6_TCP_REACHABILITY_HINTS */

/**
 * Remove all prefix, neighbor_cache and router entries of the specified netif.
 *
 * @param netif points to a network interface
 */
void
nd6_cleanup_netif(struct netif *netif)
{
    u8_t i;
    s8_t router_index;
    for (i = 0; i < LWIP_ND6_NUM_PREFIXES; i++)
    {
        if (prefix_list[i].netif == netif)
        {
            prefix_list[i].netif = NULL;
            prefix_list[i].flags = 0;
        }
    }
    for (i = 0; i < LWIP_ND6_NUM_NEIGHBORS; i++)
    {
        if (neighbor_cache[i].netif == netif)
        {
            for (router_index = 0; router_index < LWIP_ND6_NUM_ROUTERS; router_index++)
            {
                if (default_router_list[router_index].neighbor_entry == &neighbor_cache[i])
                {
                    default_router_list[router_index].neighbor_entry = NULL;
                    default_router_list[router_index].flags = 0;
                }
            }
            neighbor_cache[i].isrouter = 0;
            nd6_free_neighbor_cache_entry(i);
        }
    }
}

#if LWIP_IPV6_MLD
/**
 * The state of a local IPv6 address entry is about to change. If needed, join
 * or leave the solicited-node multicast group for the address.
 *
 * @param netif The netif that owns the address.
 * @param addr_idx The index of the address.
 * @param new_state The new (IP6_ADDR_) state for the address.
 */
void
nd6_adjust_mld_membership(struct netif *netif, s8_t addr_idx, u8_t new_state)
{
    u8_t old_state, old_member, new_member;

    old_state = netif_ip6_addr_state(netif, addr_idx);

    /* Determine whether we were, and should be, a member of the solicited-node
     * multicast group for this address. For tentative addresses, the group is
     * not joined until the address enters the TENTATIVE_1 (or VALID) state. */
    old_member = (old_state != IP6_ADDR_INVALID && old_state != IP6_ADDR_TENTATIVE);
    new_member = (new_state != IP6_ADDR_INVALID && new_state != IP6_ADDR_TENTATIVE);

    if (old_member != new_member)
    {
        ip6_addr_set_solicitednode(&multicast_address, netif_ip6_addr(netif, addr_idx)->addr[3]);

        if (new_member)
        {
            mld6_joingroup_netif(netif, &multicast_address);
        }
        else
        {
            mld6_leavegroup_netif(netif, &multicast_address);
        }
    }
}
#endif /* LWIP_IPV6_MLD */

#endif /* LWIP_IPV6 */
